PostgreSQL 9.4.1 Documentation
Prev | Up | Chapter 31. libpq - C Library | Next

# 31.2. Connection Status Functions

These functions can be used to interrogate the status of an existing database connection object.

> Tip: libpq application programmers should be careful to maintain the `PGconn` abstraction. Use the accessor functions described below to get at the contents of `PGconn`. Reference to internal `PGconn` fields using `libpq-int.h` is not recommended because they are subject to change in the future.

The following functions return parameter values established at connection. These values are fixed for the life of the `PGconn` object.

`PQdb`

  Returns the database name of the connection.

      char *PQdb(const PGconn *conn);

`PQuser`

  Returns the user name of the connection.

      char *PQuser(const PGconn *conn);

`PQpass`

  Returns the password of the connection.

      char *PQpass(const PGconn *conn);

`PQhost`

  Returns the server host name of the connection.

      char *PQhost(const PGconn *conn);

`PQport`

  Returns the port of the connection.

      char *PQport(const PGconn *conn);

`PQtty`

  Returns the debug TTY of the connection. (This is obsolete, since the server no longer pays attention to the TTY setting, but the function remains for backward compatibility.)

      char *PQtty(const PGconn *conn);

`PQoptions`

  Returns the command-line options passed in the connection request.

      char *PQoptions(const PGconn *conn);

The following functions return status data that can change as operations are executed on the `PGconn` object.

`PQstatus`

  Returns the status of the connection.

      ConnStatusType PQstatus(const PGconn *conn);

  The status can be one of a number of values. However, only two of these are seen outside of an asynchronous connection procedure: `CONNECTION_OK` and `CONNECTION_BAD`. A good connection to the database has the status `CONNECTION_OK`. A failed connection attempt is signaled by status `CONNECTION_BAD`. Ordinarily, an OK status will remain so until `PQfinish`, but a communications failure might result in the status changing to `CONNECTION_BAD` prematurely. In that case the application could try to recover by calling `PQreset`.

  See the entry for `PQconnectStartParams`, `PQconnectStart` and `PQconnectPoll` with regards to other status codes that might be returned.

`PQtransactionStatus`

  Returns the current in-transaction status of the server.

      PGTransactionStatusType PQtransactionStatus(const PGconn *conn);

  The status can be `PQTRANS_IDLE` (currently idle), `PQTRANS_ACTIVE` (a command is in progress), `PQTRANS_INTRANS` (idle, in a valid transaction block), or `PQTRANS_INERROR` (idle, in a failed transaction block). `PQTRANS_UNKNOWN` is reported if the connection is bad. `PQTRANS_ACTIVE` is reported only when a query has been sent to the server and not yet completed.

`PQparameterStatus`

  Looks up a current parameter setting of the server.

      const char *PQparameterStatus(const PGconn *conn, const char *paramName);

  Certain parameter values are reported by the server automatically at connection startup or whenever their values change. `PQparameterStatus` can be used to interrogate these settings. It returns the current value of a parameter if known, or `NULL` if the parameter is not known.

  Parameters reported as of the current release include `server_version`, `server_encoding`, `client_encoding`, `application_name`, `is_superuser`, `session_authorization`, `DateStyle`, `IntervalStyle`, `TimeZone`, `integer_datetimes`, and `standard_conforming_strings`. (`server_encoding`, `TimeZone`, and `integer_datetimes` were not reported by releases before 8.0; `standard_conforming_strings` was not reported by releases before 8.1; `IntervalStyle` was not reported by releases before 8.4; `application_name` was not reported by releases before 9.0.) Note that `server_version`, `server_encoding` and `integer_datetimes` cannot change after startup.

  Pre-3.0-protocol servers do not report parameter settings, but libpq includes logic to obtain values for `server_version` and `client_encoding` anyway. Applications are encouraged to use `PQparameterStatus` rather than ad hoc code to determine these values. (Beware however that on a pre-3.0 connection, changing `client_encoding` via `SET` after connection startup will not be reflected by `PQparameterStatus`.) For `server_version`, see also `PQserverVersion`, which returns the information in a numeric form that is much easier to compare against.

  If no value for `standard_conforming_strings` is reported, applications can assume it is `off`, that is, backslashes are treated as escapes in string literals. Also, the presence of this parameter can be taken as an indication that the escape string syntax (`E'...'`) is accepted.

  Although the returned pointer is declared `const`, it in fact points to mutable storage associated with the `PGconn` structure. It is unwise to assume the pointer will remain valid across queries.

`PQprotocolVersion`

  Interrogates the frontend/backend protocol being used.

      int PQprotocolVersion(const PGconn *conn);

  Applications might wish to use this function to determine whether certain features are supported. Currently, the possible values are 2 (2.0 protocol), 3 (3.0 protocol), or zero (connection bad). The protocol version will not change after connection startup is complete, but it could theoretically change during a connection reset. The 3.0 protocol will normally be used when communicating with PostgreSQL 7.4 or later servers; pre-7.4 servers support only protocol 2.0. (Protocol 1.0 is obsolete and not supported by libpq.)

`PQserverVersion`

  Returns an integer representing the backend version.

      int PQserverVersion(const PGconn *conn);

  Applications might use this function to determine the version of the database server they are connected to. The number is formed by converting the major, minor, and revision numbers into two-decimal-digit numbers and appending them together. For example, version 8.1.5 will be returned as 80105, and version 8.2 will be returned as 80200 (leading zeroes are not shown). Zero is returned if the connection is bad.

`PQerrorMessage`

  Returns the error message most recently generated by an operation on the connection.

      char *PQerrorMessage(const PGconn *conn);

  Nearly all libpq functions will set a message for `PQerrorMessage` if they fail. Note that by libpq convention, a nonempty `PQerrorMessage` result can consist of multiple lines, and will include a trailing newline. The caller should not free the result directly. It will be freed when the associated `PGconn` handle is passed to `PQfinish`. The result string should not be expected to remain the same across operations on the `PGconn` structure.

`PQsocket`

  Obtains the file descriptor number of the connection socket to the server. A valid descriptor will be greater than or equal to 0; a result of -1 indicates that no server connection is currently open. (This will not change during normal operation, but could change during connection setup or reset.)

      int PQsocket(const PGconn *conn);

`PQbackendPID`

  Returns the process ID (PID) of the backend process handling this connection.

      int PQbackendPID(const PGconn *conn);

  The backend PID is useful for debugging purposes and for comparison to `NOTIFY` messages (which include the PID of the notifying backend process). Note that the PID belongs to a process executing on the database server host, not the local host!

`PQconnectionNeedsPassword`

  Returns true (1) if the connection authentication method required a password, but none was available. Returns false (0) if not.

      int PQconnectionNeedsPassword(const PGconn *conn);

  This function can be applied after a failed connection attempt to decide whether to prompt the user for a password.

`PQconnectionUsedPassword`

  Returns true (1) if the connection authentication method used a password. Returns false (0) if not.

      int PQconnectionUsedPassword(const PGconn *conn);

  This function can be applied after either a failed or successful connection attempt to detect whether the server demanded a password.

`PQgetssl`

  Returns the SSL structure used in the connection, or null if SSL is not in use.

      void *PQgetssl(const PGconn *conn);

  This structure can be used to verify encryption levels, check server certificates, and more. Refer to the OpenSSL documentation for information about this structure.

  The actual return value is of type `SSL *`, where `SSL` is a type defined by the OpenSSL library, but it is not declared this way to avoid requiring the OpenSSL header files. To use this function, code along the following lines could be used:

      #include <libpq-fe.h>
      #include <openssl/ssl.h>

      ...

          SSL *ssl;

          dbconn = PQconnectdb(...);
          ...

          ssl = PQgetssl(dbconn);
          if (ssl)
          {
              /* use OpenSSL functions to access ssl */
          }

Prev | Home | Next
Database Connection Control Functions | Up | Command Execution Functions